1. Field of the Invention
The present invention relates to communication systems and more particularly to two-way telecommunication systems employing central message transmitting stations and Earth orbit relay satellites to transfer messages between mobile terminals and the central stations. The invention further relates to a two-way telecommunication system using a sequential, alternating, half duplex communication pattern to reduce complexity and power requirements for each terminal while providing reliable communications between a large number of mobile system users.
2. Background of the Art
There is an ever increasing need for communication systems to provide message delivery or brief transfers of information between a central communication office or dispatcher and a number of remote system users. There are a substantial number of commercial, governmental, and private applications requiring the delivery of relatively short messages to or from a large number of geographically dispersed terminals, or mobile transceivers, often on an irregular basis. The need for message services includes such diverse applications as government services (military, law enforcement, legislative) where secure communications are desired, forestry, resource management, disaster relief or coordination, aviation, navigation, commercial transport or transportation, and message delivery services.
Other examples include the interstate trucking industry where dispatchers wish to communicate short messages to trucks located anywhere in the continental United States, especially in rural areas. Presently the transfer of such messages is restricted to periodic telephonic communication between drivers and a central dispatcher or contact person. Currently, truck drivers must call long distance from many remote geographic locations to retrieve messages or to update delivery and pick-up schedules. However, it is hard, if not impossible, for drivers to consistently "call in" at fixed, scheduled, times since telephone services are not always readily available in many areas. The conventional "call in" approach also creates accounting problems and major personal inconvenience by requiring vehicle drivers to use money or charge cards for long distance calls at substantial cost even when no updated message or information is transferred.
Aside from conventional telephone systems, other communication systems have attempted to address the mobile market. Radio telephone, cellular telephone, and portable radio transceivers (CB) are all capable of providing some form of communication between a mobile transceiver and a base unit. However, these communication systems suffer from several drawbacks and have proven inadequate as message communication systems for serving a large number of widely dispersed users.
Current mobile communication services operate on a limited number of high frequency, low data rate, channels and have many more potential users than system capacity. Many systems, like cellular telephone, employ frequency reuse across an array of cells to increase capacity, with each cell having a short transmission range to increase frequency reuse and reduce interference. However, these lower power transmissions are more prone to frequency selective fading and signal blocking and require highly mobile users to frequently change channels as new cells are traversed. These systems are also prone to sudden communication loss when no channels are available in adjacent cells.
Cellular systems economically serve large metropolitan areas but leave many urban and most rural areas without any service or direct coverage at all. Messages for these areas are switched to conventional telephone or satellite carriers for long distance transfers to ground based systems which eliminates the flexibility desired. There are no direct satellite communication systems.
Direct communication, non-cellular, using traditional mobile radio transceivers also required constant monitoring of a variety of frequencies all of which are crowded with existing traffic. These services, like cellular telephone, are subject to frequent system overload and signal degradation from several interference sources which makes them incapable of handling a large volume of traffic. These transceivers also have a low range imposed by the need to prevent interference with other communication systems.
Communication systems based on Earth orbital relay satellites have been proposed as a new approach to solve many communication problems, especially coverage of geographically diverse, low user density or rural areas. Several systems have been proposed or developed to operate through orbital relay satellites and central communication stations. Examples of such systems are found in U.S. Pat. Nos. 4,291,409 and 4,455,651.
However, the high gain, low interference requirements imposed on such systems, especially in regard to adjacent Earth orbit relay satellites or concurrent satellite users, requires the use of large, narrow-beam, receiver antennas on the order of 4 feet or more in diameter, which eliminates mobile applications. In addition, these satellite systems are limited to low data transfer rates which severely limit the number of potential users and data transfer capacity. Otherwise, tracking and demodulating a high data rate satellite signal requires very high speed, complex, and expensive receivers that also consume substantial amounts of power for signal scanning and processing even when not receiving messages. Such equipment is impractical for a commercial message system.
What is needed is a communication system that allows continuous delivery of messages and related communication parameters or control information to a large number of users over a large geographical area. The system needs to provide a return link for message acknowledgment and responses. The system also needs to allow development of a complete private digital data network. The system needs to be cost effective; simple to operate, maintain, and install; and minimize antenna requirements. The system should also automatically adjust to handle many transfer errors associated with a mobile environment.